This invention relates to plasma channels, to display devices comprising plasma channels, and to plasma-addressed liquid crystal display panels commonly referred to as xe2x80x9cPALCxe2x80x9d display devices using such channels. PALC devices comprise, typically, a sandwich of: a first substrate having deposited on it parallel transparent column electrodes, commonly referred to as xe2x80x9cITOxe2x80x9d columns or electrodes since indium-tin oxides are typically used, on which is deposited a color filter layer; a second substrate comprising parallel sealed plasma channels corresponding to rows of the display crossing all of the ITO columns and each of which is filled with a low pressure ionizable gas, such as helium, neon and/or argon, and containing spaced cathode and anode electrodes along the channel for ionizing the gas to create a plasma, which channels are closed off by a thin transparent dielectric sheet; and a liquid crystal (LC) material located between the substrates. The structure behaves like an active matrix liquid crystal display in which the thin film transistor switches at each pixel are replaced by a plasma channel acting as a row switch and capable of selectively addressing a row of LC pixel elements. In operation, successive lines of data signals representing an image to be displayed are sampled at column positions and the sampled data voltages are respectively applied to the ITO columns. All but one of the row plasma channels are in the de-ionized or non-conducting state. The plasma of the one ionized selected channel is conducting and, in effect, establishes a reference potential on the adjacent side of a row of pixels of the LC layer, causing each LC pixel to charge up to the applied column potential of the data signal. The ionized channel is turned off, isolating the LC pixel charge and storing the data voltage for a frame period. When the next row of data appears on the ITO columns, only the succeeding plasma channel row is ionized to store the data voltages in the succeeding row of LC pixels, and so on. As is well known, the attenuation of the backlight or incident light to each LC pixel is a function of the stored voltage across the pixel. A more detailed description is unnecessary because the construction, fabrication, and operation of such PALC devices have been described in detail in the following U.S. patents and publication, the contents of which are hereby incorporated by reference: U.S. Pat. Nos. 4,896,149; 5,077,553; 5,272,472; 5,276,384; and Buzak et al., xe2x80x9cA 16-Inch Full Color Plasma Addressed Liquid Crystal Displayxe2x80x9d, Digest of Tech. Papers, 1993 SID Int. Symp., Soc. for Info. Displ. pp. 883-886.
FIG. 1 shows a flat panel display system 10, which represents a typical PALC display device and the operating electronic circuitry. With reference to FIG. 1, the flat panel display system comprises a display panel 12 having a display surface 14 that contains a pattern formed by a rectangular planar array of nominally identical data storage or display elements 16 mutually spaced apart by predetermined distances in the vertical and horizontal directions. Each display element 16 in the array represents the overlapping portions of thin, narrow electrodes 18 arranged in vertical columns and elongate, narrow channels 20 arranged in horizontal rows. (The electrodes 18 are hereinafter referred to from time to time as xe2x80x9ccolumn electrodesxe2x80x9d). The display elements 16 in each of the rows of channels 20 represent one line of data.
The widths of column electrodes 18 and channels 20 determine the dimensions of display elements 16, which are typically of rectangular shape. Column electrodes 18 are deposited on a major surface of a first electrically nonconductive, optically transparent substrate 34 (FIG. 2), and the channel rows are usually built into a second transparent substrate 36. Skilled persons will appreciate that certain systems, such as a reflective display of either the direct view or projection type, would require that only one substrate be optically transparent.
Column electrodes 18 receive data drive signals of the analog voltage type developed on parallel output conductors 22xe2x80x2 by different ones of output amplifiers 23 (FIG. 2) of a data driver or drive circuit 24, and channels 20 receive data strobe signals of the voltage pulse type developed on parallel output conductors 26xe2x80x2 by different ones of output amplifiers 21 (FIG. 2) of a data strobe or strobe means or strobe circuit 28. Each of the channels 20 includes a reference electrode 30 (FIG. 2) to which a reference potential, such as ground, common to each channel 20 and data strobe 28 is applied.
To synthesize an image on the entire area of display surface 14, display system 10 employs a scan control circuit 32 that coordinates the functions of data driver 24 and data strobe 28 so that all columns of display elements 16 of display panel 12 are addressed row by row in row scan fashion as had been described. Display panel 12 may employ electro-optic materials of different types. For example, if it uses such material that changes the polarization state of incident light rays, display panel 12 is positioned between a pair of light polarizing filters, which cooperate with display panel 12 to change the luminance of light propagating through them. The use of a scattering liquid crystal cell as the electro-optic material would not require the use of polarizing filters, however. All such materials or layers of materials which attenuate transmitted or reflected light in response to the voltage across it are referred to herein as electro-optic materials. As LC materials are presently the most common example, the detailed description will refer to LC materials but it will be understood that the invention is not limited thereto. A color filter (not shown) may be positioned within display panel 12 to develop multi-colored images of controllable color intensity. For a projection display, color can also be achieved by using three separate monochrome panels 12, each of which controls one primary color.
A partial perspective view of the PALC display described in the 1993 SID Digest is shown in FIG. 2. FIG. 2 illustrates the PALC version of such a flat display panel using LC material. Only 3 of the column electrodes 18 are shown. The row electrodes 20 are constituted by a plurality of parallel elongated sealed channels underlying (in FIG. 2) a layer 42 of the LC material. Each of the channels 20 is filled with an ionizable gas 44, closed off with a thin dielectric sheet 45 typically of glass, and contains on an interior channel surface first and second spaced elongated electrodes 30, 31 which extend the full length of each channel. The first electrode 30 is grounded and is commonly called the anode. The second electrode 31 is called the cathode, because to it will be supplied relative to the anode electrode a negative strobe pulse sufficient to cause electrons to be emitted from the cathode 31 to ionize the gas. As explained above, each channel 20, in turn, has its gas ionized with a strobe pulse to form a plasma and a grounded line connection to a row of pixels in the LC layer 42 above. When the strobe pulse terminates, and after deionization has occurred, the next channel is strobed and turned on. Since the column electrodes 18 each cross a whole column of pixels, typically only one plasma row connection at a time is allowed on to avoid crosstalk.
Fabrication of a PALC device is typically done as described in the 1993 SID digest paper by providing first and second substrates 34, 36 with the first substrate 34 comprising a glass panel on which is deposited the ITO column electrodes 18, followed by color filter processing over the ITO electrodes to produce the RGB stripes (not shown), followed by the black surround processing and liquid crystal alignment processing. The second substrate 36, also a glass panel, is masked and etched to form the channels 20, following which the plasma electrode material is deposited and masked and etched to form the cathode 31 and anode 30 electrodes. A thin dielectric glass microsheet 45 is then sealed across the peripheral edges of the device to form with the ridges 50 the channels 20, which are then exhausted, back-filled with a low-pressure ionizable gas such as helium and/or neon and optionally with a small percentage of other noble gases and sealed off. LC alignment of the exposed surface of the microsheet 45 is then carried out. The two assembled substrates are then assembled into a panel with the two LC alignment surfaces spaced apart and facing, the LC material 42 introduced into the space, and electrical connections made to the column electrodes 18 and plasma electrodes 30, 31. The method described in the referenced publication for making the plasma channels is to chemically etch a flat glass substrate to form parallel semi-cylindrically shaped recesses defined by spaced ridges or mesas and to bond on top of the mesas a thin dielectric cover sheet having a thickness in the range of about 30-50 xcexcm.
The above construction and its fabrication encounters certain problems. Since the channel electrodes must be patterned on the sloping sidewall of the channel, the dimensions and placement of the electrodes cannot be accurately controlled. Moreover, since slight variations in processing conditions can alter the etch rate, the channel etching process is difficult to control; hence the depth of the channel, which is dependent on control of the etching process, is difficult to control.
European Patent 0 500 084 A2 describes the formation of channels by patterning of electrodes on a flat substrate, providing spacers on the flat substrate, and placing the thin glass sheet on top of the spacers. The discharge space thus extends continuously across the electrodes. However, the continuous discharge space will lead between channels to crosstalk which is difficult to avoid. Moreover, the spacers have to be formed on the flat substrate by deposition and/or etching processes, such as screen printing. Since the spacers have to be as thick as the required channel depth (xcx9c100 microns or more) the fabrication of the spacers adds complexity to the process.
European Patents 0 500 085 A2 and 0554 851 A1 describe the formation of channels by screen printing partition walls. However, this is also a difficult process, which may require multiple coats to obtain the required wall height.
An object of the invention is an improved channel plate.
A further object of the invention is an improved plasma-addressed display device.
Another object of the invention is an improved method for fabricating the plasma channels of a PALC display device.
In accordance with a first aspect of the invention, a channel plate comprises a dielectric substrate and a thin dielectric sheet-like member arranged over and spaced from the substrate by a plurality of laterally spaced, channel-defining flanking wall portions each formed as part of a dielectric sheet patterned by through-holes, which latter sheet is herein referred to as the spacer sheet or plate. The holes are configured to form the desired channel configurations, typically elongated parallel channels, which preferably are straight but which also may be curved while still maintaining a substantially parallel relationship. The height of the spacer sheet above the substrate determines the height of the channels, which are each formed by the portion of the substrate surface extending between adjacent flanking wall portions, the flanking wall portions themselves forming the channel walls, and the overlying portion of the thin dielectric sheet-like member. Spaced electrodes are provided in each channel as well as a plasma-forming atmosphere. The channels are formed when the three sheet-like membersxe2x80x94the substrate, the spacer plate, and the thin dielectric sheetxe2x80x94are assembled and bonded together.
In accordance with a second aspect of the invention, the patterning of the spacer sheet is such as to provide strengthening crossbars extending preferably transverse to and between adjacent flanking wall portions. The crossbars may have a different height than that of the flanking wall portions.
In accordance with a first preferred embodiment of the invention, the substrate is of glass, the thin dielectric sheet is of glass, and the spacer sheet is a glass plate, with the through-holes in the form of slots made by chemical or plasma etching or by mechanical means such as sandblasting. The three glass members may be bonded together using fused glass frit as described in several of the cited patents and publications, or by anodic bonding as described in the first related patent application identified above.
In accordance with a another preferred embodiment of the invention, the channel plate is part of a PALC display device, and the combination of the substrate, patterned spacer plate and the overlying thin dielectric sheet-like member, together with the electrodes constitutes the plasma channels or channel plate of the PALC display device.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described the preferred embodiments of the invention, like reference numerals or letters signifying the same or similar components.